# std::ranges::lexicographical_compare

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< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
template< std::input_iterator I1, std::sentinel_for<I1> S1, std::input_iterator I2, std::sentinel_for<I2> S2, class Proj1 = std::identity, class Proj2 = std::identity, std::indirect_strict_weak_order< std::projected<I1, Proj1>, std::projected<I2, Proj2>> Comp = ranges::less > constexpr bool lexicographical_compare( I1 first1, S1 last1, I2 first2, S2 last2, Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {} ); | (1) | (since C++20)
template< ranges::input_range R1, ranges::input_range R2, class Proj1 = std::identity, class Proj2 = std::identity, std::indirect_strict_weak_order< std::projected<ranges::iterator_t<R1>, Proj1>, std::projected<ranges::iterator_t<R2>, Proj2>> Comp = ranges::less > constexpr bool lexicographical_compare( R1&& r1, R2&& r2, Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {} ); | (2) | (since C++20)

Checks if the first range `[`first1`,` last1`)` is lexicographically less than the second range `[`first2`,` last2`)`.

1) Elements are compared using the given binary comparison function comp.
2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.

Lexicographical comparison is an operation with the following properties:

- Two ranges are compared element by element.
- The first mismatching element defines which range is lexicographically less or greater than the other.
- If one range is a prefix of another, the shorter range is lexicographically less than the other.
- If two ranges have equivalent elements and are of the same length, then the ranges are lexicographically equal.
- An empty range is lexicographically less than any non-empty range.
- Two empty ranges are lexicographically equal.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

### Parameters

first1, last1 | - | the first range of elements to examine
r1 | - | the first range of elements to examine
first2, last2 | - | the second range of elements to examine
r2 | - | the second range of elements to examine
comp | - | comparison function to apply to the projected elements
proj1 | - | projection to apply to the first range of elements
proj2 | - | projection to apply to the second range of elements

### Return value

true if the first range is lexicographically less than the second.

### Complexity

At most 2·min(N1, N2) applications of the comparison and corresponding projections, where N1 = ranges::distance(first1, last1) and N2 = ranges::distance(first2, last2).

### Possible implementation

    struct lexicographical_compare_fn
    {
        template<std::input_iterator I1, std::sentinel_for<I1> S1,
                 std::input_iterator I2, std::sentinel_for<I2> S2,
                 class Proj1 = std::identity, class Proj2 = std::identity,
                 std::indirect_strict_weak_order<
                     std::projected<I1, Proj1>,
                     std::projected<I2, Proj2>> Comp = ranges::less>
        constexpr bool operator()(I1 first1, S1 last1, I2 first2, S2 last2,
                                  Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) const
        {
            for (; (first1 != last1) && (first2 != last2); ++first1, (void) ++first2)
            {
                if (std::invoke(comp, std::invoke(proj1, *first1), std::invoke(proj2, *first2)))
                    return true;

                if (std::invoke(comp, std::invoke(proj2, *first2), std::invoke(proj1, *first1)))
                    return false;
            }
            return (first1 == last1) && (first2 != last2);
        }

        template<ranges::input_range R1, ranges::input_range R2,
                 class Proj1 = std::identity, class Proj2 = std::identity,
                 std::indirect_strict_weak_order<
                     std::projected<ranges::iterator_t<R1>, Proj1>,
                     std::projected<ranges::iterator_t<R2>, Proj2>> Comp = ranges::less>
        constexpr bool operator()(R1&& r1, R2&& r2, Comp comp = {},
                                  Proj1 proj1 = {}, Proj2 proj2 = {}) const
        {
            return (*this)(ranges::begin(r1), ranges::end(r1),
                           ranges::begin(r2), ranges::end(r2),
                           std::ref(comp), std::ref(proj1), std::ref(proj2));
        }
    };

    inline constexpr lexicographical_compare_fn lexicographical_compare;

### Example

Run this code

    #include <algorithm>
    #include <iostream>
    #include <iterator>
    #include <random>
    #include <vector>

    int main()
    {
        std::vector<char> v1 {'a', 'b', 'c', 'd'};
        std::vector<char> v2 {'a', 'b', 'c', 'd'};

        namespace ranges = std::ranges;
        auto os = std::ostream_iterator<char>(std::cout, " ");

        std::mt19937 g {std::random_device {}()};
        while (not ranges::lexicographical_compare(v1, v2))
        {
            ranges::copy(v1, os);
            std::cout << ">= ";
            ranges::copy(v2, os);
            std::cout << '\n';

            ranges::shuffle(v1, g);
            ranges::shuffle(v2, g);
        }

        ranges::copy(v1, os);
        std::cout << "<  ";
        ranges::copy(v2, os);
        std::cout << '\n';
    }

Possible output:

    a b c d >= a b c d
    d a b c >= c b d a
    b d a c >= a d c b
    a c d b <  c d a b

### See also

ranges::equal (C++20) | determines if two sets of elements are the same (algorithm function object)
lexicographical_compare | returns true if one range is lexicographically less than another (function template)

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